1. Field of the Invention
This invention relates to a device for drilling rigid materials such as a stone, ceramics and concrete, and more particularly to a mist-spouting type drilling device using water or atomized water as coolant which has a sealing structure at joint portions between a rotary drill and a coolant supply pipe.
2. Description of the Prior Art
For boring holes in a rigid workpiece such as a stone, concrete, porcelain tile, glass, metal and ceramics, there has been conventionally used a drilling device using water or atomized water as coolant as illustrated in FIG. 1 as one example. The conventional drilling device has a driving means mounted within a casing 1 and operated electrically or pneumatically to rotate a hollow shaft 3. To the leading end of the hollow shaft 3, there is connected a rotary drill 5 having an axial passage 5a and a cutting bit 7 formed of ultra-hard abrasive grains such as diamond grains. The cutting bit 7 has a spouting slit 7a communicating the axial passage 5a, through which water or atomized water is discharged. The water or atomized water used as the coolant for cooling the cutting bit 7 when drilling the rigid workpiece is introduced into the inside of the hollow shaft 3 and the axial passage 5a of the drill 5 through a mist supply pipe 9 connected to the rear end of the hollow shaft 3. Though the mist supply pipe 9 must be completely connected to the hollow shaft 3 so as to prevent the coolant from leaking, this is very difficult because the hollow shaft 3 rotates at a high speed relative to the mist supply pipe 9.
A connection structure between a lubricant mist supply pipe and a hollow shaft is seen in a drilling device proposed by Japanese Patent Application Public Disclosure SHO 60-201908. In this prior art drilling device, the rotary shaft to which a drill is connected by screwing has a radial hole open to a lubricant chamber formed between the hollow shaft and an outer cylindrical casing, so as to introduce a lubricant into the drill through the lubricant chamber and the radial hole in the drill. Though the lubricant chamber is sealed by rotary seal means, sealing is insufficient and the driving efficiency is reduced because the hollow shaft rotates at a high speed relative to the outer casing. Besides, since the radial hole formed in the hollow shaft is perpendicular to the axis of the shaft, the lubricant introduced into the lubricant chamber cannot smoothly enter the hollow shaft due to the centrifugal force caused by rotating the hollow shaft.
In another drilling device proposed by European Patent Publication No. 0,252,611(B1), a coolant mist supply pipe and a hollow shaft are connected to each other by butting. The butting connection of the mist supply pipe and hollow shaft fails to completely prevent the leakage of coolant flowing therethrough when the hollow shaft rotates at a high speed. Moreover, this structure has suffered a disadvantage that counterflow of the coolant flowing into a drill retained at the leading end of the hollow shaft is brought about easily, because the flowing resistance of the coolant which is produced in drilling is increased particularly when the cutting bit provided at the leading, end of the drill moves deeply into a hole bored thereby. The counterflow of the coolant raises the pressure in the hollow shaft, thereby to easily give rise to leakage of coolant from the connecting portion between the hollow shaft and the mist supply pipe. The leakage of coolant inside the drilling device would contaminate the interior elements in the drilling device, entailing the risk of causing a leak of electricity.
Also, a drilling device proposed by U.S. Pat. No. 5,004,382 would entail the same problem. This prior art drilling device has coupling means for connecting a coolant mist supply pipe to a hollow shaft which is provided at the leading end thereof with a hollow drill. The coupling means consists of a rotary member having a circular groove and a stationary member having a circular projection which is fitted in the circular groove of the rotary member. Namely, the rotary and stationary members are in intimate face contact with each other so as to prevent leakage of coolant. As a result, the driving efficiency is remarkably reduced due to frictional resistance produced by the face contact of the rotary and stationary members. Furthermore, the problem of counterflow of the coolant as touched upon above still remains.